Slope of a line can be determined using this formula
m = (y₂ - y₁) / (x₂ - x₁)
From the question, we know that
(x₁,y₁) = (2,-3)
(x₂,y₂) = (2,9)
plug the numbers into the formula
m = (y₂ - y₁) / (x₂ - x₁)
m = (9 - (-3)) / (2 - 2)
m = (9 + 3) / (2 - 2)
m = 12/0
m = undefined
The pairs must form a vertical line